1. Field of the Invention
The present invention relates to asphalt roofing shingles, protective granules for such shingles, and processes for making such granules and shingles.
2. Brief Description of the Prior Art
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Dark blotches or streaks sometimes appear on the surfaces of asphalt shingles, especially in warmer humid climates, because of the growth of algae and other microorganisms. The predominant species responsible is Gloeocapsa sp, a blue-green algae. Other microbial growth, including fungi, moss and lichen, can also occur under proper conditions, for example, in a shady and/or persistently damp environment. In addition to being aesthetically unpleasant, the discoloration can lead to heat buildup and accelerate premature roofing failure. Eventually, severe discoloration of the entire roof can occur.
Various methods have been used in an attempt to remedy the roofing discoloration. Washing the roof surfaces with dilute cleaning solutions containing a strong oxidizer such as bleach can remove the algae from roofs. However, frequent washing and cleaning with cleaning solutions is required, since the effective duration of such treatments is rather short. In addition, topical treatments with organic algaecides have been used. However, such topical treatments are also usually effective only for short term, typically one to two years.
If the freshly cleaned surfaces are treated with a coating containing some form of biocides, the antimicrobial properties could remain for a long period of time, between five to seven years. To prevent algal growth, various types of biocides have been used. The most commonly used biocides are metals and inorganic metal oxides, such as, for example zinc metal granules and copper oxide-coated granules. However, these biocides typically persist for around ten years, and in some limited cases, for periods approaching fifteen years. One drawback is these compounds are effective against only one microbe, Gloeocapsa sp. At the same time, the service life of roofing products can extend considerably longer than ten to fifteen years, depending on the composition and structure of the roofing materials employed to construct the roof.
Companies, including Minnesota Mining and Manufacturing (3M) and ISP Mineral Products Inc., have commercialized several algaecide granules that are effective in inhibiting algae growth.
A common method used to prepare algae-resistant (AR) roofing granules generally involves two major steps. In the first step, metal oxides such as cuprous oxide and/or zinc oxide are added to a clay and alkali metal silicate mixture. The mixture in turn is used to coat crushed mineral rocks. The mixture is rendered insoluble on the rock surfaces by firing at high temperatures, such as about 500° C., to provide a ceramic coating. In the second step, the oxides covered rocks are coated with various color pigments to form colored algae-resistant roofing granules. The algae-resistant granules, alone, or in a mixture with conventional granules, are then used in the manufacture of asphalt shingles using conventional techniques. The presence of the algae-resistant granules confers algae-resistance on the shingles.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles.
U.S. Pat. No. 3,507,676 discloses roofing granules containing zinc, zinc oxide, or zinc sulfide, as an algaecide and fungicide.
Algae resistant shingles are disclosed, for example, in U.S. Pat. No. 5,356,664 assigned to Minnesota Mining and Manufacturing Co., which discloses the use of a blend of algae-resistant granules and non-algae-resistant granules. The algae-resistant granules have an inner ceramic coating comprising cuprous oxide and an outer seal coating initially devoid of copper.
There is a continuing need for algae-resistant roofing products having algaecide leaching rates that can be controlled so that the roofing products can be tailored for specific local conditions. In addition, there is a continuing need for algae-resistant roofing products that can provide sustained algae-resistance over extended periods of time.